Effect of Additive Molecular Structure on Friction Coefficient and Adsorption

Experimental results on friction coefficients obtained with a number of model additives in a four ball test apparatus have confirmed a recently proposed model of boundary lubrication. The values of adsorption free energies, calculated from friction coefficient data, agreed very closely with the free energies of adsorption determined by contact potential difference measurements. In general, the ranking order of additives based on the free energy of adsorption was the same as the published literature. The friction coefficient data obtained here were in excellent agreement with the published data on the chain length effect, chain saturation and branching and polar end-group structure. The ranking order within each group of single chain compounds was explained by the chain dispersion interactions. In general, the greater the chain dispersion interaction, the lower the minimum friction coefficient (the friction coefficient at monolayer coverage).